1. Field of the Invention
This invention relates to switchgear for electric power distribution systems, and more particularly, to heat sinks for electrical conductors in low voltage switchgear.
2. Background Information
Switchgear includes metal cabinets housing electrical apparatus and associated conductors for connecting the electrical apparatus in an electric power distribution system. The electrical apparatus can include electrical switches such as circuit breakers and network protectors which provide protection, disconnect switches for isolating parts of the electric power distribution system and transfer switches for transferring between alternate power sources. The electrical apparatus can also include metering equipment. Switchgear operating at voltages up through 600 volts are categorized as low voltage switchgear.
Typically, the electrical apparatus such as a plurality of circuit breakers is vertically stacked in a forward compartment of the switchgear cabinet. Multi-phase input and output buses must be connected to each circuit breaker. Typically these buses include a cross bus of rigid phase conductors extending transversely through the cabinet in a middle compartment behind the circuit breakers. This cross bus is connected to a vertical bus formed by phase conductors referred to as risers. The risers are connected to each circuit breaker by forwardly extending stab conductors which are connected to the circuit breaker through quick disconnects as the circuit breaker is inserted into the cabinet. The other sides of feeder circuit breakers are connected through another set of quick disconnects to runbacks which extend rearward between the risers into a rear compartment where they are connected to a cable bus system. The cable bus system and the cross bus connect the switchgear into the electric power distribution system.
These low voltage switchgear buses are designed for a range of current ratings which must meet mandated temperature rise limitations. The power circuit breakers used in such switchgear are subject to a different set of temperature rise limitations. It turns out that the temperature rise limits for the circuit breakers at the interface point between the circuit breaker disconnects and the stab conductors of the bus assemblies are about 20 degrees higher than the temperature rise limits for the buses. This temperature rise differential has been accommodated for traditionally by sizing the riser bus to adequately offset this differential along with the temperature rise effects incurred due to the bus resistance. Typically, this has been accomplished by increasing the cross-sectional area of the vertical bus appropriately.
Generally speaking, the heat contribution by power circuit breakers to the switchgear vertical bus is more severe at the higher continuous current ratings. In the past, circuit breaker frames in the lower current ratings were housed in one common size package while the frames of higher current ratings were housed in a second, larger common size package. Power circuit breakers throughout the range of current ratings provided by the assignee of this invention are housed in a single size package. This smaller package coupled with the fact that the housing is of insulated case construction which offers little to no free conductive cooling of the breaker current path internal to the housing, rather than the conventional sheet metal construction, aggravates the temperature rise problem in the risers. The common prior solution of increasing the cross-sectional area of the risers is not attractive economically due to the amount of copper required to satisfy the temperature rise requirements.
There is a need, therefore, for improved switchgear which accommodates for the higher temperatures generated by the circuit breakers and especially those with a molded case.